Alterations in the electrocardiogram after intravenous administration of dehydroemetine were studied in rats. Disturbances of intraventricular conduction and slowing of the heart rate were the earliest changes observed. Prolongation of atrioventricular conduction time and complete heart block occurred at total doses greater than 10 mg/kg. Atrial extrasystoles and paroxysmal atrial tachycardia appeared in 16% of the animals after total doses of 15 mg/kg. Ventricular extrasystoles and paroxysmal ventricular tachycardia were never observed. The cause of death was always complete heart block with ventricular asystole. The action of dehydroemetine is cumulative, since repetition of the same dose, after normal electrocardiogram had been restored, resulted in more pronounced effects.